Avoid rendering fills if empty elements inside

[youknowriad]

closes #20480

Possible regression of #19344

Steps to Reproduce:

1. Navigate to Posts > Add New
2. Insert Social Icons block
3. Click block settings
4. Observe undesirable blank section between "Duplicate" and "Remove block"

The diff looks big but in the end, it's just moving things around to avoid rendering the Fills if their content is empty. This ensures that if there are no fills, the slot is empty too.

I tried a CSS fix but it's not that easy.

[github-actions]

Size Change: +154 B (0%)

Total Size: 866 kB

Filename	Size	Change
build/block-library/index.js	116 kB	+161 B (0%)
build/editor/index.js	44.6 kB	-7 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
build/a11y/index.js	1.01 kB	0 B
build/annotations/index.js	3.43 kB	0 B
build/api-fetch/index.js	3.39 kB	0 B
build/autop/index.js	2.58 kB	0 B
build/blob/index.js	620 B	0 B
build/block-directory/index.js	6.02 kB	0 B
build/block-directory/style-rtl.css	760 B	0 B
build/block-directory/style.css	760 B	0 B
build/block-editor/index.js	104 kB	0 B
build/block-editor/style-rtl.css	10.5 kB	0 B
build/block-editor/style.css	10.5 kB	0 B
build/block-library/editor-rtl.css	7.66 kB	0 B
build/block-library/editor.css	7.66 kB	0 B
build/block-library/style-rtl.css	7.49 kB	0 B
build/block-library/style.css	7.5 kB	0 B
build/block-library/theme-rtl.css	669 B	0 B
build/block-library/theme.css	671 B	0 B
build/block-serialization-default-parser/index.js	1.65 kB	0 B
build/block-serialization-spec-parser/index.js	3.1 kB	0 B
build/blocks/index.js	57.6 kB	0 B
build/components/index.js	191 kB	0 B
build/components/style-rtl.css	15.5 kB	0 B
build/components/style.css	15.5 kB	0 B
build/compose/index.js	5.76 kB	0 B
build/core-data/index.js	10.5 kB	0 B
build/data-controls/index.js	1.03 kB	0 B
build/data/index.js	8.22 kB	0 B
build/date/index.js	5.37 kB	0 B
build/deprecated/index.js	772 B	0 B
build/dom-ready/index.js	568 B	0 B
build/dom/index.js	3.06 kB	0 B
build/edit-post/index.js	90.9 kB	0 B
build/edit-post/style-rtl.css	8.59 kB	0 B
build/edit-post/style.css	8.58 kB	0 B
build/edit-site/index.js	4.63 kB	0 B
build/edit-site/style-rtl.css	2.51 kB	0 B
build/edit-site/style.css	2.51 kB	0 B
build/edit-widgets/index.js	4.42 kB	0 B
build/edit-widgets/style-rtl.css	2.59 kB	0 B
build/edit-widgets/style.css	2.58 kB	0 B
build/editor/editor-styles-rtl.css	325 B	0 B
build/editor/editor-styles.css	327 B	0 B
build/editor/style-rtl.css	4.01 kB	0 B
build/editor/style.css	4 kB	0 B
build/element/index.js	4.45 kB	0 B
build/escape-html/index.js	733 B	0 B
build/format-library/index.js	7.6 kB	0 B
build/format-library/style-rtl.css	502 B	0 B
build/format-library/style.css	502 B	0 B
build/hooks/index.js	1.92 kB	0 B
build/html-entities/index.js	622 B	0 B
build/i18n/index.js	3.48 kB	0 B
build/is-shallow-equal/index.js	710 B	0 B
build/keyboard-shortcuts/index.js	2.3 kB	0 B
build/keycodes/index.js	1.68 kB	0 B
build/list-reusable-blocks/index.js	2.99 kB	0 B
build/list-reusable-blocks/style-rtl.css	226 B	0 B
build/list-reusable-blocks/style.css	226 B	0 B
build/media-utils/index.js	4.85 kB	0 B
build/notices/index.js	1.57 kB	0 B
build/nux/index.js	3.02 kB	0 B
build/nux/style-rtl.css	616 B	0 B
build/nux/style.css	613 B	0 B
build/plugins/index.js	2.54 kB	0 B
build/primitives/index.js	1.49 kB	0 B
build/priority-queue/index.js	780 B	0 B
build/redux-routine/index.js	2.84 kB	0 B
build/rich-text/index.js	14.3 kB	0 B
build/server-side-render/index.js	2.54 kB	0 B
build/shortcode/index.js	1.7 kB	0 B
build/token-list/index.js	1.27 kB	0 B
build/url/index.js	4 kB	0 B
build/viewport/index.js	1.61 kB	0 B
build/warning/index.js	1.14 kB	0 B
build/wordcount/index.js	1.18 kB	0 B

_{compressed-size-action}

[youknowriad]

I don't really understand where the failing 404 is being triggered.

[youknowriad]

With the new implementation, it's hard to unit test these components with snapshots. I removed the tests because these are already covered in e2e tests.

[gziolo]

I tested all the updated menu items and they still work as expected. I can also confirm that there is no empty section when there are no menu items in the plugins group.

Code wise I will leave the final call to you, @nosolosw and @getdave.